This invention relates to a process for producing a stabilized yarn for producing a carbon fiber and a thermal stabilization furnace for carrying out the process.
In a conventional industrial process for producing a carbon yarn, in order to obtain sufficient time for thermally stabilizing a continuous fiber in a furnace for producing a thermally stabilized yarn (hereinafter simply referred to as "thermal stabilization furnace"), the thermal stabilization furnace is provided with a number of rollers which are arranged either inside or outside thereof, so that the continuous yarn may travel through the furnace in a zig-zag manner, i.e., the yarn proceeds to the first roller, makes a U-turn at the first roller, travels back to the inside of the furnace, proceeds to a second roller located at the opposite side of the first roller, takes a U-turn, travel backs to the inside of the furnace and repeats this zig-zag trip.
On the other hand, in order to obtain a high strength carbon fiber, the continuous fiber must be applied with a tension while it is in a stabilizing process. In addition, it is also required that there occur no damage, fluffing, cutting of the continuous fiber, when it travels on such arranged rollers.
In order to meet with these requirements, the present applicant has proposed an invention under Japanese Patent Application No. 59(1984)-195923 which aims at preventing the occurrence of damage, fluffing, etc. on the continuous fiber. The feature of the proposed invention is in that a tension applied to the continuous fiber travelling within a thermal stabilization furnace is held constant.
Although the invention under the Japanese Patent Application No. 59(1984)-195923 can surely produce a stabilized yarn hardly susceptible to fluffing and yarn cutting, and yet having somewhat high strength, today's demand is for producing an even higher strength carbon fiber.
In order to produce a high strength carbon fiber, it is required to apply as much tension as possible to a continuous fiber and obtain a stabilized yarn having a fine radius. To this end, a larger tension, for example, near the tensile strength disclosed in the specification accompanied to the afore-mentioned Japanese Patent Application must be applied to the continuous fiber. If a tension larger than break strain is applied, there occurs as a matter of course a breakage of the continuous fiber, thereby disenabling to obtain a stabilized yarn which is higher in strength and finer in radius than the yarn described in the specification of the afore-mentioned Japanese Patent Application.